Editorial for DMOPC '17 Contest 4 P5 - Revised IOI

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Author: r3mark

The key observation to this problem is that if the four contestants answer the exact same answers to two different questions, then the answers to those two questions should be the same in the optimal answer key. Since there are only $2^4$ ~2^4~ possible ways for the contestants to answer a question, we can reduce the number of variables from $N$ ~N~ to $2^4$ ~2^4~, each of which can only be $-1$ ~-1~ or $1$ ~1~. As such, there are $2^{2^4}$ ~2^{2^4}~ answer keys which should be checked.

Time Complexity: $\mathcal O(2^{2^M}+N)$ ~\mathcal O(2^{2^M}+N)~

where $M$ ~M~ is the number of contestants (a constant in this problem).

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